#10 {km}/h = 10/3.6 m/s = 25/9 m/s# #x = v_0 * t + a * t^2/2# #=> 100 = (25/9)*t + 1.5*t^2/2# #"This is a quadratic equation in t, we solve it :"# #=> 3600 = 100*t + 27*t^2# #=> 27*t^2 + 100*t - 3600 = 0# #"discriminant : "100^2 + 3600*4*27 = 398800 = 631.5^2# #=> t = (-100 pm 631.5)/54# #"we have to take the solution with the + sign as t > 0."# #=> t = 531.5/54 = 9.843 s#